SiPMs are semiconductor photon sensitive devices made up of an array of very small Geiger-mode avalanche photodiode (APD) cells on a semiconductor substrate such as silicon. An example 10χ10 microcell array is shown in FIG. 1 of the accompanying drawings. Each cell is connected to one another to form one larger device with one signal output. The entire device size can be as small as 1χ1 mm or much larger. FIG. 2 of the accompanying drawings is a schematic diagram of a conventional silicon photomultiplier.
APD cells vary in dimension from 10 to 100 microns depending on the mask used, and can have a density of up to 3000 microcells/sq. mm. Avalanche diodes can also be made from other semiconductors besides silicon, depending on the properties that are desirable. Silicon detects in the visible and near infrared range, with low multiplication noise (excess noise). Germanium (Ge) detects infrared to 1.7 μm wavelength, but has high multiplication noise. InGaAs (Indium Gallium Arsenide) detects to a maximum wavelength of 1.6 μm, and has less multiplication noise than Ge. InGaAs is generally used for the multiplication region of a heterostructure diode, is compatible with high speed telecommunications using optical fibres, and can reach speeds of greater than Gbit/s. Gallium nitride operates with UV light. HgCdTe (Mercury Cadmium Telluride) operates in the infrared, to a maximum wavelength of about 14 μm, requires cooling to reduce dark currents, and can achieve a very low level of excess noise.
Silicon avalanche diodes can function with breakdown voltages of 20 to 500V, typically. APDs exhibit internal current gain effect of about 100-1000 due to impact ionization, or avalanche effect, when a high reverse bias voltage is applied (approximately 20-200 V in silicon, depending on the doping profile in the junction). Silicon PhotoMultipliers or SiPMs can achieve a gain of 105 to 106 by using Geiger mode APDs which operate with a reverse voltage that is greater than the breakdown voltage, and by maintaining the dark count event rate at a sufficiently low level. The current generated by an avalanche event must be quenched by an appropriate current limited scheme so that the device can recover and reset after an avalanche event.
Noise ultimately imposes a limit on the smallest signal that can be measured. The main source of noise in an SiPM is the dark count rate (DCR), which is primarily due to thermally generated electrons that go on to create an avalanche in the high field region.
The onset of non-light sensitive edge breakdown places a limit on the useful operating voltage range of the avalanche photo diode. Sufficient over voltage (biasing beyond initial breakdown) is necessary in order that avalanche probabilities achieve levels that allow a substantial PDE to be achieved. When sufficient over voltage is not possible due to the onset of non-light sensitive edge breakdown then PDE is reduced from what the device is ultimately capable of achieving. The device may be limited in operating voltage to levels that may not allow high avalanche probabilities to be achieved within the device and the architecture's full potential may not realised.
There is therefore a need to provide a semiconductor photomultiplier which addresses at least some of the drawbacks of the prior art.